According to local tradition Portmarnock is named after its patron saint Marnock:
There was a great Saint named Marnóg living on Lambay island. He had a little boat and he used to come over to the [place] which is now called Portmarnock. He had a little place for landing on [...] and the people called it Port. When families came to live in the place, they called it Port-Marnóg. The word Port means a place for landing or a port.
(SC Vol. 0792: 217)
